Bronica SQ-A 120 Back problems

I'm not positive as to why this could be happening but I am unable to switch out the back that is on my camera right now and put another one on. There is film loaded into it, the slide is in, the multiple exposure lever is off, the mirror lock up lever is in the N position...is there something else that could be preventing me from taking it off?

I haven't had this problem with mine. The safety mech, however, is operated by inserting and removing the darkslide. I suspect it is a simple mechanical latch. Do you have another back to examine? That would at least let you see how it operates, and possibly identify the fault. Alex